1. Field of the Invention
The present invention relates to a photomask technique used in microfabrication of semiconductor integrated circuits, CCDs (charge-coupled devices), color filters for LCD (liquid crystal displays), and magnetic heads.
2. Description of the Related Art
In recent years, an advanced semiconductor microfabrication technique has been an extremely important element technique to respond the requirement of finer circuit patterns accompanying high integration of large scale integrated circuits. For example, the high integration of large scale integrated circuits requires a technique for the miniaturization of wires for wiring patterns constituting circuits, or technique for forming finer contact-hole patterns for wirings between layers constituting a cell as an essential technique. The reason why the formation of finer patterns of large scale integrated circuits is accelerated is due to the high-speed operation and low power consumption thereof, and the most effective method therefor is the miniaturization of patterns.
Since most of such high degree of microfabrication is performed by photolithography technique using a photomask, the photomask has become basic techniques that support the miniaturization technique together with exposure systems and resist materials. Therefore, for the purpose of realizing a photomask having the above-described wiring patterns with finer wires or miniaturized contact-hole patterns, the development of techniques for forming finer and more accurate patterns on a photomask blank have been carried out.
The formation of a high-accuracy photomask pattern on a photomask substrate is premised on highly accurate patterning of a resist pattern formed on a photomask blank. Since photolithography in the microfabrication of a semiconductor substrate is carried out using a reduced projection method, the size of the pattern formed on the photomask is about four times the size of the pattern formed on the semiconductor substrate; however, this does not mean that the accuracy of the pattern formed on the photomask is relaxed, but the formation of the photomask pattern at higher accuracy than the pattern accuracy obtained on the semiconductor substrate after exposure is required.
At present, since the size of the circuit pattern drawn on the semiconductor substrate in photolithography is considerably smaller than the wavelength of exposure light, if a photomask on which a photomask pattern is formed by expanding the circuit pattern by four times is used as it is for reduction exposure, the shape same as the photomask pattern cannot be transferred on the resist film because of the effect of the interference of exposure light or the like.
Therefore, as a super resolution mask, an OPC mask to which a technique for correcting optical proximity effect that deteriorates transferring properties by performing a technique known as optical proximity effect correction (OPC) is applied, or a phase shift mask of which an amplitude of light at the middle of adjoining opening patterns is made zero by changing the phase of adjoining opening patterns by 180° is typically used. For example, for the OPC mask, an OPC pattern of a size ½ or less of the circuit pattern (hammer head, assist bar or the like) must be formed.
As described above, not only in photolithography for obtaining a circuit pattern on a semiconductor substrate, but also in photolithography for forming a pattern on a photomask blank, a high-accuracy patterning technique is required. One of indicators of photolithography performance is “critical resolution”, and for photolithography in the photomask patterning step, a high critical resolution equivalent to or higher than the critical resolution for photolithography in the step of circuit patterning on the semiconductor substrate is required.
Phase shift masks are roughly classified into a complete transmission (Levenson type) phase shift mask and a half-tone-type phase shift mask according to the light transmission properties of the phase shifter formed on a transparent substrate for exposure light. The light transmittance of the phase shifter formed in the complete transmission phase shift mask is substantially equal to the light transmittance of the substrate (exposed portion), and is virtually transparent to the exposure light.
In this type of photomask, since the interference effect between transmitted light of which phase is reversed by 180° from the region equipped with a phase shifter and transmitted light without phase change from the region not equipped with a phase shifter is large, excellent resolution can be obtained. However, there are problems that formable mask patterns are limited to continuous patterns, the fabrication costs of the photomask are very high, and the like.
In a half-tone phase shift mask, an absorber material having a transmittance of exposure light of some several to several tens of percent of the substrate (exposed portion) is used in the phase shifter, and although resolution as in a complete transmission phase shift mask cannot be obtained from this type of photomask, there are advantages of little restriction to the mask pattern and low fabrication costs of the photomask.
In Japanese Patent Application Laid-Open No. 2004-029746, the configuration of a photomask using the combination of a half-tone phase shift film and a transparent phase shift film is disclosed focusing respective advantages of such a complete transmission phase shift mask and a half-tone phase shift mask.
In the photomask disclosed in the above Patent Application, only one layer (lower layer phase shift film) of phase shifter (peripheral portion) is located in the periphery of the opening portion (transparent portion), and the region opposite to the opening portion of the peripheral portion is a half-tone portion wherein an upper layer phase shift film is stacked on a lower layer phase shift film. In this photomask, the transmitted light from the opening portion has the phase opposite to the phase of the transmitted light from the peripheral portion, and although phase difference adjustment is performed so that the transmitted light from the peripheral portion has the phase opposite to the transmitted light from the half-tone portion, the film forming conditions must be strictly controlled to perform such phase difference adjustment at high accuracy, causing the complication of the fabrication process and the elevation of costs. Even if phase difference adjustment is performed by the etching of the substrate in the opening portion region, such problems still remain.
In another example in the above Patent Application, a configuration wherein a phase shifter is formed by digging down the substrate by etching, and a half-tone portion is formed in the region adjacent to the phase shifter is disclosed. In this configuration, the phase shifter (peripheral portion) is separated from the opening portion (transparent portion) to form a half-tone section between them. However, even if such a configuration is formed, a high degree of processing technique is required to form the phase shifter by etching so as to cause phase change of 180° in exposure light, and the problem of the complication of the fabrication process and the elevation of costs cannot be solved.